Sounds like your Dr. is just being cautious and just making sure.
If it were something serious, he would have called you and said come in now.
So I wouldn't worry too much about it. =)
It could be atelectasis which is a condition in which the lungs or a small portion of the lung - typically at the bottom - are not fully inflated. It is a condition where the alveoli - the little ball-like sacs - are deflated. It is a very common finding in chest x-rays and is often found when other 'studies' are being done. It may be caused by normal exhalation or by several medical conditions. It can be 'acute' and happen as the result of a post-operative 'complication.'
It sounds like you're doc is being cautious, which is a good thing.